The lending unit of General Motors, which is the largest U.S. automaker, said Monday that the Department of Justice has subpoenaed documents related to its subprime lending practices.

The July 28 subpoena covers documents related to origination and securitization of subprime loans since 2007, GM Financial said in a filing Monday with the Securities and Exchange Commission. The Justice Department also is seeking information on underwriting criteria and warranties.

The inquiry by federal prosecutors adds another challenge for General Motors Chief Executive Officer Mary Barra, who is already grappling with a separate Justice Department investigation over the automaker’s slow handling of potentially fatal defects in millions of its small cars.

While that crisis has roiled for six months, GM’s sales in the United States have kept increasing with new products such as the Chevrolet Tahoe and the Buick Encore.

Easy credit has helped fuel industrywide U.S. auto sales, which are on pace for the best year since 2006.

"There’s just so much capacity being added and so much more room for easy auto credit to keep pushing ultra-low monthly payment loans on a lower and lower quality of consumer," Adam Jonas, an industry analyst with Morgan Stanley, wrote in a note to investors Monday.

"As one dealer recently told us: ‘You have to be a loser to not get 0 percent for 72 months on your car loan.’ "